This invention relates to a method of treating lake water to remove suspended solids and to precipitate and inactivate phosphorus. More particularly, this invention relates to a method of removing suspended solids and precipitating and inactivating phosphorus contained in lake water comprising adding aluminum hydroxide sulfate to the water in a concentration such that the pH of the water is maintained at a level of greater than 6.0 and precipitation of suspended solids and precipitation and inactivation of phosphorous occurs.
The use of aluminum sulfate compounds to treat lake water for the removal of suspended solids as well as the precipitation and inactivation of phosphorus compounds is well known to those skilled in the art, as illustrated, for example, by Cooke, G. D. et al., "State-of-the-Art Summary of Phosphorus Inactivation as a Lake Restoration Technique," Proceedings of Workshop on Algal Management and Control, Mar. 9-12, 1989, Pacific Grove, Calif., Technical Report E-81-7 (May 1981).
However, the use of aluminum sulfate as an additive has inherent problems when employed in lake water having low alkalinity and low pH, as aluminum sulfate tends to further depress the pH of the entire lake. For example, one mg/liter of aluminum sulfate consumes about 0.5 mg/liter of alkalinity from lake water, thereby depressing the pH of the lake. Lake pH is of particular importance because at a pH of 6.0 or less, free aluminum becomes soluble and enters the lake water. Toxicity tests have indicated that aluminum concentrations in water which are greater than about 50 .mu.g/liter are detrimental to aquatic life. Concern over the toxicity of aluminum in lake water is well-known to those skilled in the art, as illustrated, for example, by Gibbons, M. V. et al., "Effects of Multiphase Restoration, Particularly Aluminum Sulfate Application, on the Zooplankton Community of a Eutrophic Lake in Eastern Washington," Journal of Freshwater Ecology, Vol. 2, pp. 393-404 (April 1984).
To alleviate the above-described problem, sodium aluminate has been used in conjunction with aluminum sulfate. The use of sodium aluminate typically involves detailed testing to determine the pH adjustment required to negate the low pH phenomenon typical of aluminum sulfate addition. In practice, sodium aluminate is typically added to maintain lake water at a neutral pH (7.0). In the past, this has been accomplished by the addition of aluminum sulfate followed by sodium aluminate; however, this method still promotes brief episodes of toxic conditions. In addition, overdosing of sodium aluminate could cause high pH excursions (greater than a pH of 8.0) and resolubilization of the aluminum ion. Although recent equipment modifications have allowed for the simultaneous addition of aluminum sulfate and sodium aluminate, differences in product densities and application rates make operating the equipment difficult, slows the addition process, and increases the treatment costs. It would therefore be advantageous if a substitute for the use of aluminum sulfate or aluminum sulfate in combination with sodium aluminate could be found which effectively removes suspended solids and cause the precipitation and inactivation of phosphorus from lake water.
It is one object of this invention to provide a method of treating lake water to remove suspended solids and to precipitate and inactivate phosphorus by adding aluminum hydroxide sulfate to the water. It is a feature of this invention that the use of aluminum hydroxide sulfate will provide a cost effective and efficient additive for use in treating lake water as compared with using aluminum sulfate in combination with sodium aluminate. The method of this invention is advantageous in that a single additive may be used to remove suspended solids and precipitate and inactivate phosphorus contained in lake water while maintaining the pH of the water at a level of greater than 6.0, thereby avoiding the introduction of toxic amounts of soluble aluminum into the water.